AFAIK the only way is to boot into recovery mode from the XP CD and copy the file from the CD to the correct location on the HDD.

However, in my experience when explorer.exe (the Windows kernel) has problems like this a virus or other malware has caused the issue, unless the user deleted those useless *.dll files to make more space available (Don't laugh I've seen it done). You really are better off using SystemRescueCD, or similar, to back up all important data and starting from a formatted hard drive. Scan all files on a separate PC with up to date AntiVirus before putting them back onto the laptop.

Perhaps it's pointless pointing out that this would be better asked on a Windows forum.

Anyway trying to bring it back to Linux, I would try getting an Explorer.exe onto a USB key from another system, then boot your mates PC from a live Linux distro and use it to copy the Explorer.exe from the USB key.

TBH I'd be surprised if this was the limit of the problem, I don't think Windows would let him delete Explorer.exe so it's likely that something nasty has happened which would probably have taken out other stuff as well and you could end up jumping through hoops replacing one file after another.

It will probably be better and easier to reinstall (preferably Linux, but reinstall XP if you must.)

The 1st couple of issues with the Laptop was its increadibly Slooooow speed
Tried a windows de-frag, but the drive had 1% space free.
(1 Gig of temp internet files and all his data on the puny C: drive.
I dont think its been cleared or backed up since I last did it over a year ago!)

So I backed up his docs + transferred them to the empty D: partition.

But I need to stress how painfully slow this Lappys (not-going).
Anyway it locked up again (for the 6th time that day). I had to do a hard reset.

After a fs check by windows, said there were a few corrupted files and then booted as normal. Thats when XP decided to deny all knowledge of explorer...Aaarrgghhh.

My main issue is now going to be, can I retrieve the XP key from the laptop, as I wont be able to re-use mine. When I do the repair-re-install?

@ MartyBartfast =
I understand this is a Linux forum, and I've also posted on a PC forum too.
But I just thought I would chance my hand. (But I dont wanna annoy anyone
or outstay my welcome. Just thought I'd try).

I was actually thinking along the lines of what MartyBartfast mentioned,
and installing Ubuntu.

I gave it a quick try and apart from running very slowly in the live cd mode
the only major prob I'm now having is trying to partition the drive.
(It seems Ubuntu doesnt want me to mess with the current structure.
so I cant add a swap partition).

Just choose "Manual partitioning" during the installation and delete all partitions if you are going to install Ubuntu. Then create "/", "swap" and "/home" partitions and continue with the installation. Choosing to automatically "Use entire hard disk" will also get rid of the offending OS

Talking of Win laptops running out of space, colleague of mine has a very well known manufacturers product with two partitions, c,d. It seems his pc runs an automatic back-up prog on a regular basis, backing up everything on c, which is the only drive in use, to d. No problem with that except it seems not to delete previous back-ups as it goes. You can guess the rest! Quite a bit of fuss about it on some Win forums so I hear. All tech support says is 'Its bad to delete anything'. Ho hum.....................